This invention relates to electrophotographic photoreceptors. More particularly, this invention relates to electrophotographic photoreceptor charge transport layers and methods of making them.
Photoreceptors are useful in electrophotographic imaging systems, especially xerographic imaging systems, and typically contain a substrate, a conductive layer, a charge blocking layer, a charge generating layer, and a charge transport layer. Other layers are often also included.
The charge transport layer typically contains an activating small molecule dispersed or dissolved in a polymeric film forming binder. For example, charge transport layers composed of aryl diamines dispersed in polycarbonates are known in the art. Reference is made, for example, to U.S. Pat. No. 4,265,900. Charge transport layers composed of carbonatearylamine polymers are also known in the art. Reference is made, for example, to U.S. Pat. Nos. 4,806,444; 4,935,487; and 4,806,443, all to Yanus et al.
Generally, solvents are used to apply charge transport polymer layers to the surface of the photoreceptor. However, volatile organic solvents are undesirable because their emissions are harmful to the environment. Therefore, it is continually desirable to avoid the use of such solvents.
Conventional organic photoreceptors are made by solution coating techniques using preformed polymers. Reference is made, for example, to the Yanus et al. patents listed above. The use of preformed polymers poses several restrictions, including: 1) the use of solvents, which is undesirable for the reasons given above; 2) choice of solvents is limited by polymer solubility; 3) solution concentration is limited by polymer solubility and viscosity; 4) maximum polymer molecular weight is limited by required coating viscosity; and 5) the polymer cannot be crosslinked, which significantly limits its usefulness in liquid ink applications.
Although excellent toner images may be obtained with multilayered belt photoreceptors that are developed with dry developer powder (toner), it has been found that these same photoreceptors become unstable when used with liquid development systems. These photoreceptors suffer from such problems as cracking and crazing in critical charge transport layers during belt cycling; crystallization of the activating molecules in the charge transport layers; and phase separation of activating compounds. In addition, extraction of activating compounds caused by contact with the organic carrier fluid, isoparaffinic hydrocarbons, e.g., Isopar, or normal paraffinic hydrocarbons, e.g., Norpar.RTM. hydrocarbons, commonly used in liquid developer inks, markedly degrades the mechanical integrity and electrical properties of the photoreceptor.
Cracks developing in the charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality.